1. Field of the Invention
The present invention relates to a gradient index lens suitable as a collimator lens for a semiconductor laser or light-emitting diode or as a pickup lens for an optical disk.
2. Related Background Art
A SELFOC (trade name) lens is known as a lens with a refractive index profile in a direction perpendicular to an optical axis thereof, i.e., as a gradient index lens of a so-called radial type, and is widespread as an erect equal-size image focusing element in a copying machine or the like.
It is possible to prepare compact gradient index lenses. Gradient index lenses have been recently used in a variety of applications such as collimator lenses for semiconductor lasers or light-emitting diodes or as pickup lenses for optical disks. Such lenses have a large NA (i.e., Numerical Aperture) in practice and must have optical characteristics near diffraction limits even if an object is not located on its axis but near it. The prescribed characteristics are obtained by providing the radial type gradient index lens with arcuate two end faces, as described in Japanese Patent Laid-open Nos. 122512/1983 and 62815/1984. The main reason for using the gradient index lens, however, lies in the fact that the two end faces of the gradient index lens can be easily polished to obtain flat surfaces as compared with a small spherical lens requiring spherical polishing. Therefore, it is undesirable to perform spherical polishing of the two end faces of the gradient index lens.